1. Field of the Invention
The present invention relates to a method of growing a single crystal of a compound semiconductor.
2. Description of the Related Art
The known technique for vertically growing a single crystal using a crucible includes a Vertical Bridgeman method (VB method) and a Vertical Gradient Freeze method (VGF method). In these known methods, a seed 12 is disposed in the lowermost portion of a crucible 11 accommodating a melt 10 prepared by the heating of a compound semiconductor material with a multi-split heater 13, as shown in FIG. 1. In this case, a temperature gradient is vertically arranged along the melt 10 so that the uppermost portion of the melt 10 has the highest temperature, and the melt temperature gradually decreases toward the lowermost portion in the vicinity of the seed 12, as shown in FIG. 2. Under the condition, the melt crystallizes, starting with the lowermost portion. In the conventional technique, the lower end portion 14 of the crucible is downward tapered, in a funnel-form, in order to suppress the twin generation in the initial stage of the crystal growth. In other words, the diameter in the lower end portion of the crucible gradually converges on the seed 12. Specifically, where an angle .THETA. shown in FIG. 1, i.e., a set angle between the lower end portion 14 of the crucible 11 and the horizontal line is 54.4.degree., a (111) facet plane tends to appear in the shoulder portion 15 of the crystal in the case of using a seed having a crystal orientation (001). The presence of the facet plane causes twin boundaries to be generated easily inside the crystal. To cope with the defect, the set angle .THETA. at the lower end portion of the crucible is reduced to be smaller than 54.4.degree..
However, in the case of growing a crystal of a large diameter, i.e., 2 inches or more, from the seed portion by the conventional technique outlined above, a twin tends to be frequent in the vicinity of the seed 12. In addition, there is room for further improvement on the conventional technique in respect of a temperature distribution which is to be applied to the crystal within the crucible 11, with the result that it is difficult to obtain a single crystal of a large diameter directly growing from the seed portion.